The present invention relates to the short range wireless transmission of data.
Connectionless (i.e., wireless) transmission of analog and digital signals has previously been accomplished using:
Radio (including LF, HF, VHF, SHF and microwave links);
Optical (such as laser or IRxe2x80x94used in most television remotes and IRDA);
Acoustics (usually ultrasound);
Capacitive coupling; and
Magnetic coupling
Radio transmissions diminish relatively slowly with distance. They are also subject to fades and wave interference. Radio transmissions are highly regulated as to their frequencies and radiated energy. Furthermore, as radio transmissions radiate energy, they require significant power.
Optical transmissions are typically highly directional in nature. They are also stopped by objects in the transmission path. This directional aspect is often double edged; the directional aspect is often desirable to avoid interference, but this makes orientation and line of sight considerations important, and is often cumbersome. Also, these systems require significant power.
Ultrasound transmission systems are expensive and also use significant power.
The wireless data transmission method or the invention is distinct from those listed above, but is more closely associated with Capacitive coupling and Magnetic coupling, as the transmission is dominated by near field interaction as opposed to the far field radiation of the transmission methods which rely on the propagation of electromagnetic waves to transmit data.
Magnetic coupling systems are known in which a transmitter generates a varying magnetic field which can be detected by a receiver in order to transmit data. Examples include U.S. Pat. No. 3,898,565, to Takeuchi et al, U.S. Pat. No. 5,437,057 to Richley et al., and U.S. Pat. No. 5,771,438 to Palermo et al. However, these systems all require a significant power source in order to generate the magnetic field.
In U.S. Pat. No. 5,796,827 to Coppersmith et al, PCT application Ser. No. 96/36134 to Gershenfeld et al., and in a an article by T. G. Zimmerman, xe2x80x9cPersonal Area Networks: Near-field intrabody communicationxe2x80x9d, IBM Systems Journal vol 35, Numbers 3 and 4, 1996, pages 1-9, data transmission is accomplished by close range capacitive coupling of a person to the receiver electrode. When the electric potential (or voltage) of the person is modulated with respect to ground, a voltage (or current depending on the detector details) is induced on the receiver electrode with respect to ground. The ground is required to complete the circuit between the transmitter and receiver in this system. While useful for particular applications, the method requires some coupling of the user to ground, for the signal at the receiver is measured between the capacitive pickup electrode and ground. While useful in many applications that provide a suitable coupling to ground, the technique is not applicable to transmitters that are at a floating potential with respect to the receiver ground.
Thus, there exists a need for a wireless transmission system which overcomes the limitations of the above described techniques.
It is an object of the present invention to provide a low range, low power wireless transmission system. Advantageously, such a system can be built at low cost.
The invention comprises a wireless data link using the spatial gradient of quasi-static electric potentials as the medium. By quasi-static, we mean that the time variance in the electric potential is sufficiently small that electrostatic assumptions can be made. In other words, even though the electric potentials are deliberately varied in time (in order to transmit the data), any electromagnetic waves which are produced by such time variances in the electric potentials can be effectively ignored (e.g., the amount of radiated energy is well below the limits established by the FCC), and electrostatic assumptions can be made. Another feature of the quasi-static nature of the electric potentials discussed herein is that the distance between the transmitter and receiver is very much less than the ratio of the speed of light to the operating frequency (i.e. very much less than the wavelengths corresponding to the maximum frequencies of the transmission). However, the distance between the transmitter and receiver can be large compared to the size of the transmitting and receiving electrodes (the xe2x80x9cantennaexe2x80x9d). Devices using the invention can operate in base band, or they can transmit a modulated carrier signal. Amplitude, frequency and phase modulation can be used.
The transmitter creates an electric scalar potential field with a time varying spatial gradient and the receiver is sensitive to the spatial gradient of this potential. In most practical applications, the transmitter will form a time varying dipole electric potential field by varying the voltage on electrodes of a given geometry. The receiver will sense the spatial gradient of the electric potential field by measuring the signal induced between two or more electrodes separated by some distance. This induced signal can be the current needed to hold the receiver electrodes at a constant potential. However, as an alternative the receiver can measure the voltage induced between the receive electrodes.
The invention is especially advantageous for low cost and low power applications which require short range, low data rate transmission. However, the invention is not limited to these type of systems, as various design trade-offs can be made between the range, data rate, cost, antenna size and power consumption parameters.
In accordance with a broad aspect of the present invention there is provided a method of data transmission comprising:
varying a voltage difference applied across a plurality of transmit electrodes at such a rate that the resulting electric potential field varies in a quasi-static manner, wherein the variation in said voltage difference depends on the data to be transmitted; and
producing a receive output signal which varies with a signal induced between a plurality of receive electrodes by said quasi-static variations in said electric potential field.
In accordance with another aspect of the present invention there is provided a wireless transmission system comprising:
a wireless transmitter; and
a wireless receiver,
wherein said transmitter comprises:
a plurality of transmit electrodes separated in space;
means for varying a voltage difference across said electrodes at a rate that the resulting electric potential field varies in a quasi-static manner, wherein the variation in said voltage difference depends on the data to be transmitted; and
wherein said receiver comprises:
a plurality of receive electrodes separated in space,
means for producing a receive output signal which varies with a signal induced between said receive electrodes by the varying quasi-static electric potential field produced by said transmit electrodes.
In accordance with yet another aspect of the present invention there is provided a transmitter for transmitting data by slowly varying the spatial gradient of an electric potential field comprising:
at least two transmit electrodes;
means for changing the voltage potential between said electrodes to vary the spatial gradient of an electric potential field generated by said transmitter;
means for controlling said means for changing in order to vary said spatial gradient to transmit data.
In accordance with yet another aspect of the present invention there is provided a receiver comprising:
at least two electrodes separated in space; and a detector connected to said electrodes for producing a signal which varies as the spatial gradient of the electric potential field across the receive electrodes varies.